Light-emitting element and method for producing the same

ABSTRACT

A light-emitting element includes a sapphire substrate including: a principal surface that is in a c-plane of the sapphire substrate, and a plurality of projections on the principal surface, wherein each of the plurality of projections has a shape of pseudo-hexagonal pyramid including six lateral surfaces, each of the six lateral surfaces including an inwardly curved surface portion, and wherein, in a top view of the sapphire substrate, each of the plurality of projections has a shape of a pseudo-hexagon that includes first curved lines and second curved lines that are alternately connected to one another, the first curved lines being curved toward a center of a corresponding hexagon and disposed between respective adjacent pairs of six vertices of the hexagon, and the second curved lines passing through respective vertices of the hexagon; and a semiconductor layered body comprising a nitride semiconductor on the principal surface side of the sapphire substrate, the semiconductor layered body including an active layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2015-231118 filed on Nov. 26, 2015. The entire disclosure of JapanesePatent Application No. 2015-231118 is hereby incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a light-emitting element including asubstrate having projections and to a method for producing thelight-emitting element.

2. Description of Related Art

Generally, light-emitting elements (e.g., light-emitting diodes: LEDs)made of semiconductors such as nitride semiconductors each has astructure in which an n-type semiconductor layer, an active layer, and ap-type semiconductor layer are layered on a sapphire substrate in thisorder. Techniques of forming projections on sapphire substrates toimprove light extraction efficiency of light-emitting elements have beenproposed. (e.g., see WO 2011/074534).

SUMMARY

According to a producing method in WO 2011/074534, hexagonal-shapedresists are formed on a sapphire substrate using an exposure mask, andthe sapphire substrate is etched using the resists, which formshexagonal-pyramid shaped projections.

In the producing method in WO 2011/074534, etching is performed usingresists each having a simple hexagonal shape. Since corner portions of aresist tend to be more quickly etched off, the hexagonal-shaped resistsmay be deformed into near-circular shapes as the etching progresses.Thus, the projections may each have a near-conical shape. Accordingly,with the technique disclosed in WO 2011/074534, the projections may eachhave a shape similar to a cone rather than hexagonal pyramids. Certainembodiments of the present invention have been made in view of such aproblem. One object of certain embodiments of the present invention isto provide a light-emitting element with improved light extractionefficiency, and a method for producing the light-emitting element.

A light-emitting element according to an embodiment of the presentinvention includes a sapphire substrate and a semiconductor layeredbody. The sapphire substrate has a principal surface that is a c-planeof the sapphire substrate. The semiconductor layered body is made ofnitride semiconductors and disposed on a principal surface side of thesapphire substrate. The semiconductor layered body includes an activelayer. The sapphire substrate has a plurality of projections on theprincipal surface. Each of the plurality of the projections has a shapeof a pseudo-hexagonal pyramid. The pseudo-hexagonal pyramid includes sixlateral surfaces. Each of the six lateral surfaces includes an inwardlycurved surface portion. Each of the projections has a shape of apseudo-hexagon in a top view. The pseudo-hexagon includes first curvedlines curved toward the center of a hexagon and second curved linespassing through the vertices of the hexagon. The first curved lines aredisposed between six vertices of the hexagon. The second curved linesand the first curved lines are connected to each other.

A method for producing a light-emitting element according to anembodiment of the present invention includes disposing a plurality ofresists on a principal surface of a sapphire substrate, forming aplurality of projections on the principal surface by dry-etching thesapphire substrate, growing semiconductor layers made of nitridesemiconductors on a principal surface side of the sapphire substrate togrow a semiconductor layered body including a light-emitting layer. Theprincipal surface is a c-plane of the sapphire substrate. Thedry-etching of the sapphire substrate removes the resists. In thedisposing of the resists, each of the resists is formed to have apseudo-hexagonal shape in a top view. The pseudo-hexagon includes firstcurved lines obtained by curving six sides of a hexagon toward thecenter of the hexagon and second curved lines passing through verticesof the hexagon. The second curved lines are connected to the firstcurved lines.

In the light-emitting element according to an embodiment of the presentinvention, crystal growth on the lateral surfaces of the projections canbe reduced, which allows for forming semiconductor layers containingless gaps or voids, so that light extraction efficiency can be improved.In the method of producing a light-emitting element according to anembodiment of the present invention, the resists are formed to haveshapes with consideration of ease of etching, the pseudo-hexagonalpyramid projections projecting from the sapphire substrate and havingnear-hexagonal bottom surfaces can be formed, and a light-emittingelement having improved light extraction efficiency can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a light-emitting elementaccording to a first embodiment.

FIG. 2A is a schematic unit cell diagram illustrating plane orientationsof a sapphire crystal in a sapphire substrate.

FIG. 2B is a schematic plan view of a sapphire crystal structureillustrating the plane orientations of the sapphire crystal in thesapphire substrate.

FIG. 3 is a schematic plan view of part of an alignment of projectionson the sapphire substrate in the light-emitting element according to thefirst embodiment.

FIG. 4 is a schematic perspective view of the projections of thesapphire substrate in the light-emitting element according to the firstembodiment cut along the line IV-IV in FIG. 3.

FIG. 5A is an enlarged schematic plan view of one of the projections ofthe sapphire substrate in the light-emitting element according to thefirst embodiment.

FIG. 5B is a schematic cross-sectional view of the projection of thesapphire substrate in the light-emitting element according to the firstembodiment cut along the line VB-VB in FIG. 5A.

FIG. 6A is a schematic plan view for illustrating the case where a ratioof the diameter of the circumscribed circle to the diameter of theinscribed circle of the projection of the sapphire substrate in thelight-emitting element according to the first embodiment is 1.125.

FIG. 6B is a schematic plan view illustrating the case where the ratioof the diameter of the circumscribed circle to the diameter of theinscribed circle of the projection of the sapphire substrate in thelight-emitting element according to the first embodiment is 1.23.

FIG. 6C is a schematic plan view for illustrating the case where theratio of the diameter of the circumscribed circle to the diameter of theinscribed circle of the projection of the sapphire substrate in thelight-emitting element according to the first embodiment is 1.35.

FIG. 6D is a schematic plan view for illustrating the case where theratio of the diameter of the circumscribed circle to the diameter of theinscribed circle of the projection of the sapphire substrate in thelight-emitting element according to the first embodiment is 1.5.

FIG. 7 is a flowchart illustrating an exemplary method of producing thelight-emitting element according to the first embodiment.

FIG. 8 is a schematic exploded perspective diagram illustrating theshapes of a mask and resists for forming the projections on the sapphiresubstrate in the light-emitting element according to the firstembodiment.

FIG. 9 is a schematic plan view of the resists for forming theprojections on the sapphire substrate in the light-emitting elementaccording to the first embodiment.

FIG. 10A is a schematic diagram illustrating a state in which theresists have been disposed on the sapphire substrate in thelight-emitting element according to the first embodiment.

FIG. 10B is a schematic diagram illustrating a state in which thesapphire substrate in the light-emitting element according to the firstembodiment is etched via the resists.

FIG. 10C is a schematic diagram illustrating a state in which thesapphire substrate in the light-emitting element according to the firstembodiment and the resists on the sapphire substrate have been etched,and the projections have been formed.

FIG. 11 is a schematic plan view of a part of an alignment of theprojections projecting from the sapphire substrate in the light-emittingelement according to a second embodiment.

FIG. 12 is a schematic exploded perspective diagram illustrating theshapes of a mask and resists for forming the projections on the sapphiresubstrate of the light-emitting element according to the secondembodiment.

FIG. 13 is a schematic plan view of the resists for forming theprojections on the sapphire substrate in the light-emitting elementaccording to the second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

A light-emitting element according to each embodiment will be describedbelow referring to the drawings. The drawings referred in thedescriptions below schematically illustrate each embodiment. The scales,the distances, the positional relations, or the like of members may beexaggerated, or illustration of part of the members may be omitted insome cases. Also, in the descriptions below, the same term or referencenumeral generally represents the same member or members made of the samematerial, and its detailed description will be omitted as appropriate.Furthermore, directions in each drawing is not intended to representabsolute positions but represents relative positions of components.

First Embodiment

An example of a light-emitting element according to a first embodimentwill be described referring to FIG. 1 to FIG. 6D.

As shown in FIG. 1, a light-emitting element 1 includes a sapphiresubstrate 10, a buffer layer 20, and a semiconductor layered body 30made of nitride semiconductors. As an example, the light-emittingelement 1 including an n-side electrode 40, a p-side electrode 50, and alight-transmissive electrode 60 is described below.

The sapphire substrate 10 is a member for supporting and growing thesemiconductor layered body 30. The semiconductor layered body 30 isformed by layering semiconductor layers each made of a nitridesemiconductor. The sapphire substrate 10 includes a plurality ofprojections 11 at a predetermined interval in a top surface of thesapphire substrate 10 on a c-plane side, which is a principal surfaceside of the sapphire substrate 10. Also, the sapphire substrate 10 hasan overall thickness including the projections 11, for example, in therange of 300 μm to 1,000 μm.

As shown in FIG. 2A and FIG. 2B, the sapphire substrate 10 is made of asapphire crystal SC, which has a hexagonal crystal structure, and theprincipal surface of the sapphire substrate is the c-plane (i.e., (0001)plane). The c-plane in the present specification may have an off angle,that is, may be slightly inclined with respect to the c-plane. Thedegree of the off angle is, for example, about equal to or less than 3°.The sapphire crystal SC has, in addition to the c-plane, six m-planesand three a-planes. The m-planes are lateral surfaces of a hexagonalprism in a unit cell diagram, and are respectively perpendicular to them₁-axis, the m₂-axis, and the m₃-axis. The a-planes include a firsta-plane SA1, a second a-plane SA2, and a third a-plane SA3 that areperpendicular to the a₁-axis, the a₂-axis, and the a₃-axis,respectively.

The projections 11 enable the semiconductor layered body 30 containingless crevices or voids to be formed in the case where a crystal of thesemiconductor layers is grown on the sapphire substrate 10. As shown inFIG. 3 to FIG. 5B, each of the projections 11 has a pseudo-hexagonalpyramid shape having six lateral surfaces 12 each including an inwardlycurved surface portion. The projection 11 has a pseudo-hexagonal topview shape (i.e., a shape of the bottom surface) formed by couplingfirst curved lines curved toward the center of an imaginary hexagon anddisposed between six vertices 13 of the imaginary hexagon and secondcurved lines passing through the vertices 13 of the imaginary hexagon ina top view. Furthermore, in the pseudo-hexagon, curved line portionsbetween the vertices 13 are referred to as sides 14, and the sides 14are curved lines each inwardly curved at the midpoint 16 of the lengthof a side 14. In other words, the projection 11 has a pseudo-hexagonalpyramid shape having a bottom surface with a pseudo-hexagonal shape. Thethick lines SA1 to SA3 in FIG. 3 are lines drawn to imaginarilyrepresent the directions of the planes.

Thus, the shape of the projection 11 in a top view is made of aplurality of curved lines, so that surfaces of the projection 11 aremade of curved surfaces having comparatively large curvatures. In thecase where the projections each have a conventional cone shape, alateral surface of each of the projections has relatively smallcurvature. Accordingly, in the lateral surface of each of suchprojections, areas of the r-planes and their approximate planes, onwhich crystals are easily grown, tend to be increased. In the case wherethe projections each have a pyramid shape, the projections are requiredto be disposed in specific orientations so that the lateral surfacesdoes not coincide with the r-planes or their approximate planes. This issimilar in the case where the projections each has a shape such thatcorners of pyramids are rounded off. In each of the projections 11according to the present embodiment, however, the lateral surfaces aremade of the curved surfaces having relatively large curvatures, whichcan reduce areas of the r-planes and their approximate planes in thelateral surfaces in any orientation of arrangement of the projections.In other words, the lateral surfaces of each of the projections 11according to the present embodiment are mainly made of planes other thanr-planes and their approximate planes, on which crystals are easilygrown from the lateral surfaces of the projection 11, of the sapphiresubstrate 10. That is, the lateral surfaces of each of the projections11 according to the present embodiment are mainly made ofcrystal-growth-suppressed surfaces. With this structure, unintendedcrystal growth from the lateral surfaces 12 of each of the projectionscan be suppressed.

If unintended crystal growth of a semiconductor occurs on theprojections on the sapphire substrate 10, growth of the semiconductorlayer from a crystal growth plane located between the projections mayeasily be hindered, which may lead to difficulty in stable growth of thesemiconductor layer. Accordingly, crystal defects such as dislocationsmay be generated in the semiconductor layered body 30 to be formed, or agap or a void may be generated in the semiconductor layered body 30 andthus light extraction efficiency may be reduced. In the presentembodiment, however, lateral surfaces of each of the projections 11 aremade mainly of crystal-growth-suppressed planes to suppress unintendedcrystal growth, which allows the semiconductor layer to be stably grownfrom the crystal growth surface. With this structure, a semiconductorlayer having a surface containing less dislocations can be formed. Thatis, in the present embodiment, with the lateral surfaces having thestructure described above, a semiconductor crystal is not easily grownfrom the lateral surfaces 12 and can be stably grown from the crystalgrowth surface (the c-plane) between the projections 11, so thatgeneration of a gap or a void around the projections 11 can be reduced.Thus, with the present embodiment, generation of a gap or a void in thesemiconductor layered body 30 can be reduced, which can suppressoccurrence of problems described above, so that a light-emitting elementhaving good light extraction efficiency can be obtained.

In the present embodiment, the projections 11 are arranged so that thesides of the imaginary hexagon that is the original hexagon of thepseudo-hexagon is parallel to the a-planes (i.e., (11-20) planes) of thesapphire substrate 10. In this case, the r-planes (i.e., (−1012) planes)intersect with the sides of the imaginary hexagon at a 30-degree angleand are inclined with respect to the c-plane at about 58 degrees. Thatis, in the lateral surfaces of each of the protrusions, the r-planes andtheir approximate planes are located in the vicinity of ridge lines Rsof each of the projections 11. Meanwhile, the ridge lines Rs are formedby curved surfaces having relatively large curvatures and being coupledto the lateral surfaces 12. Accordingly, even if the lateral surfacesincludes portions of the r-planes or their approximate planes, locationof the regions of the r-planes and their approximate planes are limitedto be in a portion of the ridge lines Rs. The other portion of thesurfaces of each of the projections 11 are crystal-growth-suppressedsurfaces, which can reduce unintended crystal growth from the lateralsurfaces 12 of the projection, so that generation of a gap or a void canbe suppressed, and thus a light-emitting element having good lightextraction efficiency can be obtained.

The projections 11 are each formed as the pseudo-hexagonal pyramid sothat each of the six continuous lateral surfaces 12 is a curved surfacethat is inwardly curved at the center of the lateral surface 12 and islocated between the ridge lines Rs that are each located betweenadjacent lateral surfaces 12. Herein, each of the ridge lines Rs locatedbetween adjacent lateral surfaces 12 is a portion having maximumcurvature in the curved surface continuously formed between the adjacentlateral surfaces 12. With the lateral surfaces 12 that are such curvedsurfaces, growth of a semiconductor layer from the lateral surfaces 12can be delayed, and the semiconductor layer can be stably grown from thecrystal growth surface (i.e., c-plane). In a top view, each of thelateral surfaces 12 has an approximate isosceles triangle shape made ofa side 14 and ridge lines Rs adjacent to each other with a top portion15 being one vertex. The base (i.e., side 14) of the lateral surface 12is inwardly curved at its center toward the center of the top view ofthe each of the protrusions 11, which is defined by projecting the topportion 15 onto a bottom surface side. Furthermore, within the plane ofthe lateral surface 12, the curvature between adjacent ridge lines Rs isformed to be the maximum on the base (side 14) and become graduallysmaller toward the top 15.

In each drawing, the ridge lines Rs are represented by thin linesbetween adjacent lateral surfaces 12 of the pseudo-hexagonal pyramid,which are schematic illustrations for easy understanding of thesefigures. Also, the six vertices 13 forming the bottom surface of each ofthe projections 11 are represented as points at which diagonals passingthrough the center of an imaginary regular hexagon and dividing the areaof the imaginary regular hexagon into six equal parts are intersectedwith curved lines protruding outward among the curved lines forming thebottom surface of each of the projections 11.

In each of the projections 11, a circumscribed circle C1 of thepseudo-hexagon passing through the vertices 13 serves as the outermostdiameter portion of the pseudo-hexagon being the bottom surface of eachof the projections 11, and an inscribed circle C2 of the pseudo-hexagonpassing through the midpoints 16 of the sides of the pseudo-hexagonserves as the innermost diameter portion of the pseudo-hexagon being thebottom surface of each of the projections 11. The ratio of the diameterr1 of the circumscribed circle C1 to the diameter r2 of the inscribedcircle C2 is preferably in a range of 1.1 to 1.8. With the ratio of thediameter r1 of the circumscribed circle C1 to the diameter r2 of theinscribed circle C2 that is 1.1 or greater, the bottom surface of theprojection 11 can be prevented from having a circular shape, so thatdistances between the projections 11 can be easily made even. Also, withthe ratio of the diameter r1 of the circumscribed circle C1 and thediameter r2 of the inscribed circle C2 that is 1.8 or smaller,difference between a distance from the top portion 15 to the vertices 13and a distance from the top portion 15 to the midpoints 16 of the curvedsides 14 are prevented from being too large, and difficulty in formingof the semiconductor layered body 30 having a reduced dislocationdensity can be avoided. This is because increase of difference betweenthe distance from the top portion 15 to the vertices 13 and the distancefrom the top portion 15 the midpoints 16 of the curved sides 14 causesdecrease of the area of the projection 11 in the top view compared withthe case where the positions of the vertices 13 are not changed and thedifference between the distance from the top portion 15 to the midpoints16 and the distance from the top portion 15 to the vertices 13 isreduced. With this structure, the area of the crystal growth surface,that is, a flat surface (c-plane) on which the projections 11 are notformed is relatively increased.

Accordingly, with this structure, an effect of reducing dislocations dueto the projections 11 can be less easily obtained. The ratio describedabove is more preferably in the range of 1.1 to 1.3 and furtherpreferably in the range of 1.1 to 1.25 in order to enhance lightextraction efficiency. In other words, since the ratio of the diameterof the circumscribed circle C1 to the diameter of the inscribed circleC2 of a regular hexagon is about 1.15, the pseudo-hexagonal pyramiddesirably has a pseudo-hexagonal base having a shape with a ratio closeto this ratio of the regular hexagon. In the present embodiment, thepseudo-hexagon that is the shape of the bottom surface of thepseudo-hexagonal pyramid has rounded corners, so that the pseudo-hexagonmay have a shape with a value smaller than the ratio of the regularhexagon, 1.15.

FIG. 6A to FIG. 6D show examples of the shape of the bottom surface ofeach of the projections 11 of the sapphire substrate 10. A projection 11shown in FIG. 6A has a shape similar to the projection 11 shown in FIG.5A, and is formed so that the ratio of the diameter of the circumscribedcircle C1 to the diameter of the inscribed circle C2 is about 1.12. In aprojection 11B shown in FIG. 6B, a ratio of the diameter of thecircumscribed circle C1 to the diameter of the inscribed circle C2thereof is about 1.23. Compared with the projection 11, this projection11B has larger curvatures and larger degrees of curve of sides 14B, andthe inclined angles of lateral surfaces 12B are steeper. In a projection11C shown in FIG. 6C, a ratio of the diameter of the circumscribedcircle C1 and the inscribed circle C2 is about 1.35. Compared with theprojection 11B, this projection 11C has even larger curvatures andlarger degrees of curve of sides 14C, and the inclined angles of lateralsurfaces 12C are steeper. Accordingly, in the lateral surfaces 12C ofthe projection 11C, areas containing crystal growth surfaces (r-planesand their approximate planes) of the sapphire substrate 10 is reduced.

Furthermore, in a projection 11D shown in FIG. 6D, a ratio of thediameter of the circumscribed circle C1 to the diameter of the inscribedcircle C2 is about 1.5. Compared with the projection 11C, thisprojection 11D has even larger curvatures and larger degrees of curve ofsides 14D, and the inclined angles of lateral surfaces 12D are steeper.Accordingly, in the lateral surfaces 12D of the projection 11D, areascontaining crystal growth surfaces (r-planes and their approximateplanes) of the sapphire substrate 10 is reduced. The lateral surfaces ofthe projections 11B to 11D are made of curved surfaces similarly to theprojection 11, on which semiconductor layers having a good crystallinitycan be formed, so that light-emitting elements having good lightextraction efficiency can be obtained. Herein, the r-planes of thesapphire substrate refers to the (−1012) plane, and the approximateplanes of the r-planes refers to planes deviated from the r-planes byabout 1 to 2 degrees or less. The surfaces of the projection 11 arepreferably formed of planes other than such planes in view of reducingcrystal growth.

As shown in FIG. 3 and FIG. 4, the tops 15 of the projections 11 arepreferably arranged in a triangular lattice pattern in a top view. Inthe case where the tops 15 (i.e., the centers of the projections 11 in atop view) are arranged in a triangular lattice pattern, the projections11 are disposed so that a side of one of the projections 11 is parallelto one of the sides 14 of an adjacent projection 11. Also, in the casewhere the tops 15 of the projections 11 are arranged in a triangularlattice pattern in a top view, the projections 11 are further preferablyarranged in an equilateral triangular lattice pattern. With thisarrangement, distances between adjacent projections 11 can be even in asurface of the sapphire substrate 10. Accordingly, the growth rate ofthe semiconductor layer is substantially uniform within the crystalgrowth surface, so that a semiconductor layer having a goodcrystallinity can be easily formed, which can improve the lightextraction efficiency of the light-emitting element 1. Also, arrangementof the projections 11 in a triangular lattice pattern enables aplurality of projections 11 to be densely arranged. The distance (in thedescription below, referred to as a distance between the projections)between the tops 15 of adjacent projections 11 is preferably in therange of 2.2 to 3.1 μm, for example. With such distance between theprojections, the crystallinity of the semiconductor layer to be grown onthe sapphire substrate 10 can be improved, which can improve the lightextraction efficiency of the light-emitting element 1 to be obtained.

Also, with the projections 11, the number of dislocations to begenerated on the surface of the nitride semiconductor can be reduced. Amechanism of reducing the number of dislocations will be describedbelow. In the sapphire substrate 10, each of the lateral surfaces 12forming the projections 11 of the substrate is mainly made of acrystal-growth-suppressed plane, which suppresses growth of the nitridesemiconductor thereon. On the other hand, a portion of a surface of thesapphire substrate 10 located between adjacent projections 11 is, forexample, made of the c-plane, which is a crystal growth surface on whichthe nitride semiconductor can be grown. When the nitride semiconductoris grown in the vertical direction (i.e., thickness direction of thesapphire substrate 10) from the crystal growth surface, dislocations dueto the difference between the lattice constant of the sapphire substrate10 and the lattice constant of the nitride semiconductor tend to extendin the growing direction and to appear on a surface of the nitridesemiconductor formed. When the nitride semiconductor is grown on thesapphire substrate 10, the nitride semiconductor is grown from thecrystal growth surface located between the projections 11, while growthfrom the lateral surfaces 12 of the projections 11 is suppressed and thenitride semiconductor is not grown substantially. Accordingly, suchstructure can reduce dislocations appearing on the surface of thenitride semiconductor. For example, the dislocation density in a portionnear a light-emitting layer can be about 5×10⁷ dislocations/cm⁻² to5×10⁸ dislocations/cm⁻².

In the case where the semiconductor layered body 30 is formed by crystalgrowth of GaN, a crystal of GaN, which is in a hexagonal system, isgrown with an upper direction being the c-axis direction. In lateraldirections, the growth hardly progresses in the m-axis directionscompared with in the a-axis directions. Thus, crystals tends to be grownwith facet planes whose bases being planes equivalent to the m-planes(i.e., planes perpendicular to the c-plane of the sapphire substrate 10)of GaN in a plan view. In this case, the m-planes of GaN are locatedalong the same planes as the a-planes of the sapphire substrate 10. Inother words, GaN tends to be grown with facets whose bases being linescoinciding with the a-planes of the sapphire substrate 10 in a top view.

Accordingly, in a surface of the sapphire substrate 10, the projections11 are preferably disposed so that the sides 14 of the pseudo-hexagonalpyramid will be parallel to planes different from the m-planes of thesapphire substrate 10, preferably parallel to the a-planes of thesapphire substrate 10. That is, as shown in FIG. 3, the projections 11are preferably formed so that the sides 14 of the bottom faces will beparallel to the a-planes of the sapphire substrate 10. The expression“the sides 14 being parallel to the a-planes” refers to that, whenadjacent vertices 13 are connected by straight lines, the straight linesare parallel to any one of the a-planes (i.e., any of the first a-planeSA1 to the third a-plane SA3).

With the projections 11 disposed so that the sides 14 of thepseudo-hexagon will be parallel to any one of the first a-plane SA1, thesecond a-plane SA2, and the third a-plane SA3, the bases of the facetplanes of the semiconductor layered body 30 will be parallel to theouter peripheries of the sides 14 of the pseudo-hexagonal pyramid ofeach of the projections 11. With such structure of the projections 11,the nitride semiconductor is grown in the lateral directions to coverthe projections 11 as the growth progresses, so that the dislocationsextending in the growing direction are trapped within the nitridesemiconductor. Accordingly, dislocations present on the surface of thenitride semiconductor can be reduced, and the semiconductor layered body30 having a low dislocation density can be formed. That is, the nitridesemiconductor of which crystals grown in the upper direction and thelateral directions gradually covers the projection 11 as the growthprogresses. Finally, portions of the nitride semiconductor grown fromthe entire perimeter of each of the projections 11 are integrated oneach of the projections 11. At this time, with the projection 11 havinga structure described above, the nitride semiconductor tends to convergeat one point (e.g., at a point near the center of the projection 11) onthe projection 11. Accordingly, crystal defects extending in the growingdirection tend to converge at one point or tend to be trapped within thenitride semiconductor, so that dislocations present on a surface of thenitride semiconductor layer can be reduced.

As shown in FIG. 1, the buffer layer 20 is provided in order toaccommodate difference in lattice constants between the sapphiresubstrate 10 and the semiconductor layered body 30 to be grown on thesapphire substrate 10. The buffer layer 20 is formed between thesapphire substrate 10 and the semiconductor layered body 30. The bufferlayer 20 is made of, for example, AlN or AlGaN. The buffer layer 20 canbe formed by performing sputtering, for example, under predeterminedconditions at a step of arranging the buffer layer as described below inthe producing method. The buffer layer 20 has, for example, a layershape covering the sapphire substrate 10 as shown in FIG. 1, while thesapphire substrate 10 may be partly exposed from the buffer layer 20. Asdescribed below, the buffer layer 20 is more preferably made ofsingle-crystal AlN formed by sputtering. With such material, the numberof dislocations appearing on the surface of the semiconductor layeredbody 30 can be further reduced. Thus, a light emitting element usingsuch material for the buffer layer 20 can have the values of, forexample, the output, Vf (i.e., forward voltage), and temperatureproperties that are better than the values of these of a light-emittingelement using AlGaN for the buffer layer 20.

The semiconductor layered body 30 made of nitride semiconductorsconstitutes a light-emitting portion of the light-emitting element 1.Examples of the nitride semiconductor include In_(X)Al_(Y)Ga_(1-X-Y)N(0≤X, 0≤Y, and X+Y≤1). As shown in FIG. 1, the semiconductor layeredbody 30 is formed on a c-plane side (i.e., principal surface side) ofthe sapphire substrate 10 with the buffer layer 20 disposedtherebetween, and includes an n-side semiconductor layer 31, an activelayer 32, and a p-side semiconductor layer 33 that are layered in thisorder from the principal surface side of the sapphire substrate 10. Theactive layer 32 has a quantum well structure including, for example, awell layer (light-emitting layer) and a barrier layer. On thesemiconductor layered body 30, for example, the n-side electrode 40 isformed on a portion of the n-side semiconductor layer 31 exposed byremoving a portion each of the active layer 32 and the p-sidesemiconductor layer 33, and the p-side electrode 50 is formed on thep-side semiconductor layer 33. The p-side electrode 50 may be disposedon the light-transmissive electrode 60 formed on substantially theentire surface of the p-side semiconductor layer 33.

Method for Producing Light-Emitting Element

Next, a method of producing the light-emitting element according to thefirst embodiment will be described referring to FIG. 7 to FIG. 10C.

In the method of producing the light emitting element according to thefirst embodiment, first, as shown in FIG. 7, a step S1 of disposing aresist film on the sapphire substrate 10 is performed. The resist filmis formed on the entire surface of the sapphire substrate 10. The resistfilm is then exposed to light through a mask Ma (see FIG. 8), therebyperforming an exposing step S2 of forming a resist pattern of resists Raeach having a predetermined shape. The number of the predeterminedshapes correspond to the number of the projections 11 to be formed onthe principal surface (c-plane) side of the sapphire substrate 10. Afterthat, a step S3 of dry-etching the sapphire substrate 10 via the resistsRa (see FIG. 8) each having the predetermined shape formed in thelight-exposing step S2 to form the projections 11 on the c-plane of thesapphire substrate 10 is performed.

Furthermore, a step S4 of disposing the buffer layer 20 on the sapphiresubstrate 10 with the projections 11 is performed. After that, a step S5of growing a semiconductor layer on the buffer layer 20 to form thesemiconductor layered body 30 is performed, so that the light-emittingelement 1 is produced.

In the step S1 of disposing the resist film, the resist film is disposedon the sapphire substrate 10, for example, by application using a spincoater. In the case where the step S1 is performed on the sapphiresubstrate 10, before disposing the resist film, ashing is performed toslightly roughen the surface of the substrate by hitting a surface ofthe sapphire substrate 10 with plasma in order that the resist film tobe formed will be less likely to be detached. Also, after the ashing,cleaning such as hydro cleaning is performed to remove dirt adhered to asurface of the substrate. The step S1 of disposing the resist film onthe sapphire substrate 10 is then performed on the c-plane (0001) of thecleaned sapphire substrate 10. Examples of the resist film includepositive resist films containing novolac resins as a main component. Theashing and the cleaning are not essential and may be omitted asappropriate in order to simplify the production procedure.

After disposing the resist film on the sapphire substrate 10, as shownin FIG. 8, the exposing step S2 of exposing the resist film is performedby irradiation with light (i.e., irradiation with light includingultraviolet light) through the mask Ma having a predetermined shape. Inthe exposing step S2, the mask Ma having such a shape that can patternthe resist film into resists Ra that can form the projections 11 eachhaving a desired pseudo-hexagonal pyramid shape after the etching of thesapphire substrate 10 is used.

The mask Ma used in the light-exposing step S2 has a plurality throughholes Ha each having, in a plan view, a pseudo-hexagonal shape formed bycoupling first curved lines and second curved lines. The first curvedlines are formed by curving six sides of a hexagon toward the center ofthe hexagon. The second curved lines pass through the vertices of thehexagon, as shown in FIG. 8. In more detail, the through hole Ha has ashape made of the first curved lines and the second curved lines, inwhich the first curved lines are formed by inwardly bending the sides ofa regular hexagon between the vertices of a regular hexagon at themidpoints of the sides, which serve as apexes of the first curved lines,and the second curved lines are curved toward the vertices of theregular hexagon, which serve as the apexes of the second curved lines.The pseudo-hexagonal shapes of the through holes Ha in the mask Ma areused for patterning into the resists Ra on the sapphire substrate 10.Via the mask Ma, the exposing operation is performed using a stepper orother devices, so that the resists Ra serving as etching masks withpseudo-hexagonal resist patterns is formed, as shown in FIG. 9. At thistime, the resists Ra have a top view shape that are approximately thesame as the shapes of the through holes Ha formed in the mask Ma. Afterperforming the exposing step S2, developing and UV curing are performed,which allow the resists to be formed into the pseudo-hexagons and baked,so that the resists Ra that can be used as the etching masks in theetching are formed (see FIG. 10A).

In the step S3 of dry-etching the sapphire substrate 10 via the resistsRa, the sapphire substrate 10 is dry-etched via the resists Ra formed inthe light-exposing step S2 described above to form the projections 11 onthe sapphire substrate 10 by the dry-etching. In the step S3 ofdry-etching, the sapphire substrate 10 and the resists Ra are eroded toform the projections 11 having desired shapes.

The step S3 of dry-etching will be described referring to FIG. 10B andFIG. 10C. After the etching is started, at an early stage, the etchingremoves portions of the sapphire substrate 10 on which the resists Raare not formed, so that shapes approximately the same with the shapes ofthe mask are reflected to shapes of the remaining portions of thesapphire substrate 10. As the etching progresses, the sapphire substrate10 is subjected to effects of anisotropy in an etching rate (i.e.,variance in a rate at which the etching progresses depending ondirections) due to forms of crystals, and is formed into shapesreflecting the forms of crystals while the resists Ra are graduallyremoved (see FIG. 10B). More specifically, due to variance in theetching rate depending on etching progress directions, the etchingprogresses while reflecting the crystal form, and thus six vertices 13at a bottom portion and ridge lines of the pseudo-hexagonal pyramidgradually become definite, so that protrusions corresponding to thepseudo-hexagonal-pyramid shaped projections are formed on the sapphiresubstrate 10 below corresponding pseudo-hexagonal resists Ra.

Each of the protrusions to be formed into the pseudo-hexagonal pyramidhas a flat top surface while the resists Ra are present on the topsurface (see FIG. 10B). Also, the periphery of the each of theprotrusions is etched into a pseudo-hexagonal shape, and the area ofeach of the protrusions become smaller than the area of each of theresists Ra. Portions of each of the protrusions corresponding to thelateral surfaces 12 and the sides 14 of each of the projections 11 areformed into inwardly curved arc shapes. The portions of each of theprotrusions corresponding to the lateral surfaces 12 of each of theprojections 11 are formed into approximate isosceles triangles with thesides 14 serving as the bases of the isosceles triangles. As the etchingprogresses, the areas of these portions are reduced, and the curvatureradius of the arc of each of sides 14 is increased.

As the etching further progresses, the resist Ra disposed on the topsurface of each of the protrusions are eroded, which gradually reducesthe area of the resist Ra, and then, when completed, the resist Radisappears. This allows for gradually forming a portion corresponding tothe top 15 of the projection 11. The protrusion is thus formed into thepseudo-hexagonal pyramid having a sharp top 15 of the projection 11 (seeFIG. 10C).

In the step S3 of dry-etching, the resist Ra has a pseudo-hexagonal topview shape made of the first curved line and the second curved lineconnected to each other, in which the first curved lines are formed bycurving six sides of a hexagon toward the center of the hexagon, andsecond curved lines pass through the vertices of the hexagon. Morespecifically, the resists Ra is formed into the pseudo-hexagonal shapein a top view in which the portions corresponding to the vertices 13 ofthe shape of the projection 11 to be formed protrude toward the outercircumference in the diagonal directions of the hexagon. With thisshape, the vertices 13 of each of the projections 11 forming thepseudo-hexagonal pyramid can prevent forming a shape of a bottom surfacethe projection 11 to be a near-circular shape like the bottom surface ofa cone. In other words, the ease of erosion by etching of the resist Rais estimated to select the shape of the resist Ra in a top view of theprojections 11 to be formed. More specifically, since protrudingportions of the resist Ra tend to be more easily eroded, each of theprotrusions 11 has a top view in which protruding portions of the shapeof the projection 11 in a top view is further protruded. The shape ofthe resist Ra in a top view can be selected by determining relationshipbetween conditions of a resist and the pseudo-hexagonal pyramid to beformed through experiments or simulations.

In the step S3 of dry-etching, in a top view, each of the resists Rapreferably has a shape in which the ratio of the diameter of acircumscribed circle C3, which passes through a portion to be theoutermost circumference of the pseudo-hexagon, to the diameter of aninscribed circle C4, which passes through a portion to be the innermostcircumference of the pseudo-hexagon, is in a range of 1.3 to 2.0. Withthe ratio of the diameters of the circumscribed circle C3 and theinscribed circle C4 of 1.3 or more, the shape of the bottom surface ofeach of the projections 11 to be formed can be prevented from beingcircular. Also, with the ratio of the diameters of the circumscribedcircle C3 and the inscribed circle C4 of 2.0 or smaller, great increaseof the proportion of the areas of crystal growth surfaces within asurface of the sapphire substrate 10 can be prevented, so that effectsof suppressing dislocations by the projections 11 can be efficientlyachieved.

More specifically, vapor-phase etching, plasma etching, or reactive ionetching can be used for the dry etching. Examples of an etching gas usedfor the etching include Cl-based and F-based gases such as Cl₂, SiCl₄,BCl₃, HBr, SF₆, CHF₃, C₄F₈, and CF₄, and inert gases such as Ar.Examples of resists that can be used in the dry etching include positiveresist films containing novolac resins as a main component.

With the above-described method of processing the sapphire substrate,the sapphire substrate 10 with projections 11 on which a semiconductorlayer having good crystallinity can be grown can be obtained. In thestep of disposing a semiconductor layered body 30 described below, usingthe processed sapphire substrate 10 having the projections 11 allows asemiconductor layered body 30 having a good crystallinity to be formed,so that the light-emitting element 1 having good light extractionefficiency can be obtained.

After the step S3 of dry etching, the substrate is cleaned and dried,and then the step S4 of forming the buffer layer 20 on the sapphiresubstrate 10 on which the projections 11 have been formed is performed.The buffer layer 20 is made of AlN, AlGaN, or other materials, andpreferably formed using AlN. In the step S5 of disposing thesemiconductor layered body 30 described below, growing the semiconductorlayer on the buffer layer 20, which is made of single-crystal AlN andformed by sputtering, can form a semiconductor layer having goodcrystallinity on the c-plane of the sapphire substrate 10. Meanwhile, inthis case, crystals tend to be grown on inclined surfaces of theprojections 11. In the present embodiment, however, the lateral surfaces12 of each of the projections 11 are made of crystal-growth-suppressedsurfaces, the growth of crystals on the inclined surfaces can besuppressed even in the case where the buffer layer 20 is made of AlN, sothat a semiconductor layer having a good crystallinity can be formed.

Next, the step S5 of disposing the semiconductor layered body 30including the light-emitting layer by growing semiconductor layers onthe sapphire substrate 10 is performed. The semiconductor layers areformed by growing nitride semiconductors.

With the sapphire substrate 10 on which the projections 11 have beenformed, in the step S5 of disposing the semiconductor layered body 30,the number of dislocations in the semiconductor layered body 30 can beeasily reduced, as described above. Furthermore, the lateral surfaces 12of each of the projections 11 are made of crystal-growth-suppressedsurfaces, which can reduce generation of unintended crystals, so thatthe semiconductor layer can be stably grown. Accordingly, thesemiconductor layered body 30 having good crystallinity can be formed,so that a light-emitting element having good light extraction efficiencycan be produced. With the shapes and the arrangement of the projections11 as described above, dislocations of the crystals of the semiconductorlayered body 30 can be converged in a shorter time than in conventionalmethods, so that the crystals can be grown to form a flat surface at aposition near the sapphire substrate 10. Also, thecrystal-growth-surface region of the sapphire substrate 10 locatedbetween adjacent projections 11 can have a uniform width. Thisarrangement allows the growth rate of the semiconductor layer to beconstant, so that a semiconductor layer having a good crystallinity canbe obtained.

Second Embodiment

Next, a second embodiment will be described. The second embodimentdiffers from the first embodiment in the shape of the mask, the shape ofthe resist, and the arrangement of the projections 11. The otherconstitutions that have been already described are the same, and theirdescriptions will be omitted as appropriate. The thick lines SA1 to SA3in FIG. 11 are imaginary lines drawn to represent the directions of thea-planes.

As shown in FIG. 11, the projections 11 formed on the sapphire substrate10 are arranged so that vertices 13 of two adjacent projections 11 willface each other. In other words, each of the vertices 13 of oneprojection 11 is disposed near a vertex 13 of an adjacent projection 11.Furthermore, the projections 11 are arranged so that the tops 15 thatare the centers of the projections form a triangular lattice pattern ina top view. On the substrate 10, any one of the sides 14 of theprojections 11 are formed to be parallel to any one of the a-axes of thesapphire substrate 10.

Also, on the substrate 10, any one of the sides of the imaginaryhexagons of the projections 11 are formed to be perpendicular to any oneof the a-planes of the sapphire substrate 10. With this arrangement, ther-planes and their approximate planes, on which crystals are easilygrown, of the sapphire substrate are mainly located near three of thelateral surfaces 12 of the projection. In the present embodiment, thethree of the lateral surfaces 12 are made of curved surfaces.Furthermore, in a top view, each of the projections has a shapeincluding first curved lines curved toward the center of a hexagon anddisposed between six vertices of the hexagon, and second curved linespassing through the vertices of the hexagon. The first curved lines andthe second curved lines are coupled to each other. With this shape, thelateral surfaces 12 of each of the projections each has an inclinationangle steeper than in the case where the projections each has a conicalor near-conical shape, which can reduce the areas of the r-planes andtheir approximate planes of the sapphire substrate present in thelateral surfaces 12 of each of the projection. Accordingly, unintendedcrystal growth can be reduced.

A mask Mb as shown in FIG. 12 is used in order to form the projections11 on the sapphire substrate 10 so that vertices 13 of adjacentprojections 11 are aligned so as to face each other (i.e., so as to belocated at the shortest distance). Through holes Hb are formed in themask Mb. By exposing the resist film using a stepper or other devicesthrough the mask Mb as described above, resists Rb are formed as shownin FIG. 13. The sapphire substrate 10 is then dry-etched using theformed resists Rb as the etching mask, so that the projections 11 can beformed on the c-plane of the sapphire substrate 10. As shown in FIG. 11,the projections 11 are disposed so that the sides 14 will be alignedwith respect to the a-planes of the sapphire substrate 10.

In each of the projections 11, the ratio of the diameter r1 of thecircumscribed circle C1 to the diameter r2 of the inscribed circle C2 ofthe pseudo-hexagon is selected in the range of 1.1 to 1.8.Configurations in the present embodiment is similar to the constitutionof the first embodiment described above except that the orientation ofthe pseudo-hexagon in the present embodiment is rotated at an angle of30 degrees with respect to the pseudo-hexagon in FIG. 6A to FIG. 6Ddescribed above. Adjustment of the ratio to be in the predeterminedrange described above can be realized by adjusting the protrudinglengths and widths of portions of the mask Mb corresponding to thevertices 13 of the pseudo-hexagon.

The second embodiment as described above can have effects similar to theeffects of the first embodiment described above.

What is claimed is:
 1. A light-emitting element comprising: a sapphiresubstrate including: a principal surface that is in a c-plane of thesapphire substrate, and a plurality of projections on the principalsurface, wherein each of the plurality of projections has a shape ofpseudo-hexagonal pyramid including six lateral surfaces, each of the sixlateral surfaces including an inwardly curved surface portion, andwherein, in a top view of the sapphire substrate, each of the pluralityof projections has a shape of a pseudo-hexagon that includes firstcurved lines and second curved lines that are alternately connected toone another, the first curved lines being curved toward a center of acorresponding hexagon and disposed between respective adjacent pairs ofsix vertices of the hexagon, and the second curved lines passing throughrespective vertices of the hexagon; and a semiconductor layered bodycomprising a nitride semiconductor on the principal surface side of thesapphire substrate, the semiconductor layered body including an activelayer.
 2. The light-emitting element according to claim 1, wherein aratio of a diameter of a circumscribed circle of the pseudo-hexagon to adiameter of an inscribed circle of the pseudo-hexagon is in a range of1.1 to 1.8.
 3. The light-emitting element according to claim 1, whereinat least one side of the hexagon of each of the plurality of projectionsis parallel to an a-plane of the sapphire substrate in the top view. 4.The light-emitting element according to claim 2, wherein at least oneside of the hexagon of each of the plurality of projections is parallelto an a-plane of the sapphire substrate in the top view.
 5. Thelight-emitting element according to claim 2, wherein the plurality ofprojections are arranged such that, in the top view, a side of thehexagon of a first of the plurality of projections is parallel to a sideof the hexagon of a second of the plurality of projections adjacent tosaid first projection.
 6. The light-emitting element according to claim4, wherein the plurality of projections are arranged so that, in the topview, a side of the hexagon of a first of the plurality of projectionsis parallel to a side of the hexagon of a second of the plurality ofprojections adjacent to said first projection.
 7. The light-emittingelement according to claim 1, wherein centers of the plurality ofprojections are arranged in a triangular lattice pattern in the topview.
 8. The light-emitting element according to claim 2, whereincenters of the plurality of projections are arranged in a triangularlattice pattern in the top view.
 9. The light-emitting element accordingto claim 1, further comprising: a buffer layer made of AlN, the bufferlayer disposed on the principal surface of the sapphire substrate,wherein the semiconductor layered body is disposed on the buffer layer.10. A method of producing a light-emitting element, the methodcomprising: disposing a plurality of resists on a principal surface of asapphire substrate, the principal surface being in a c-plane of thesapphire substrate; forming a plurality of projections on the principalsurface by dry-etching the sapphire substrate via the resists, thedry-etching removing the resists; and disposing a semiconductor layeredbody on a principal surface side of the sapphire substrate by growingsemiconductor layers comprising nitride semiconductors, thesemiconductor layered body including a light-emitting layer, wherein, inthe disposing of the plurality of resists, each of the plurality ofresists is formed such that, in a top view of the sapphire substrate,each of the plurality of resists has a shape of a pseudo-hexagon thatincludes first curved lines and second curved lines that are alternatelyconnected to one another, the first curved lines being curved toward acenter of a corresponding hexagon and disposed between respectiveadjacent pairs of six vertices of the hexagon, and the second curvedlines passing through respective vertices of the hexagon.
 11. The methodof producing a light-emitting element according to claim 10, wherein, inthe disposing of the plurality of resists, centers of the plurality ofpseudo-hexagons are arranged in a triangular lattice pattern.
 12. Themethod of producing a light-emitting element according to claim 10,wherein, in the disposing of the plurality of resists, eachpseudo-hexagon is disposed such that a side of the hexagon is parallelto an a-plane of the sapphire substrate.
 13. The method of producing alight-emitting element according to claim 11, wherein, in the disposingof the plurality of resists, each pseudo-hexagon is disposed such that aside of the hexagon is parallel to an a-plane of the sapphire substrate.14. The method for producing a light-emitting element according to claim10, wherein, in the disposing of the plurality of resists, the resistsare arranged such that a side of the hexagon of a first of theprojections is parallel to a side of the hexagon of a second of theprojections adjacent to said first projection.
 15. The method forproducing a light-emitting element according to claim 12, wherein, inthe disposing of the plurality of resists, the resists are arranged sothat a side of the hexagon in one of the projections to be parallel to aside of the hexagon in another one of the projections adjacent to saidfirst projection.
 16. The method for producing a light-emitting elementaccording to claim 10, further comprising: between the forming of theprojections and the disposing of the semiconductor layered body,disposing a buffer layer on the principal surface of the sapphiresubstrate, the buffer layer being made of AlN, wherein the semiconductorlayers are grown on the buffer layer.
 17. The method for producing alight-emitting element according to claim 11, further comprising:between the forming of the projections and the disposing of thesemiconductor layered body, disposing a buffer layer on the principalsurface of the sapphire substrate, the buffer layer being made of AlN,wherein the semiconductor layers are grown on the buffer layer.
 18. Themethod for producing a light-emitting element according to claim 10,wherein, in the disposing of the plurality of resists, each of theplurality of resists is disposed such that a ratio of a diameter of acircumscribed circle of the pseudo-hexagon to a diameter of an inscribedcircle of the pseudo-hexagon in a range of 1.3 to 2.0.
 19. The methodfor producing a light-emitting element according to claim 11, wherein,in the disposing of the plurality of resists, each of the plurality ofresists is disposed such that a ratio of a diameter of a circumscribedcircle of the pseudo-hexagon to a diameter of an inscribed circle of thepseudo-hexagon in a range of 1.3 to 2.0.